This invention relates to electrical data acquisition and control systems and more particularly to methods and apparatus for remotely monitoring and calibrating signal conditioning circuits for such systems.
Modern process plants and other complex automated systems require the gathering of data from a large number of sources throughout the installation. The data gathered typically includes various analog signals such as temperatures, pressures, flows, fluid levels et cetera, and digital signals which indicate a condition or status such as whether a pump is off or on, or whether a valve is opened or closed et cetera. The signals are generated by sensors located throughout the plant and transmitted by field wiring to central locations where the signals are used in control systems, monitoring systems and/or protective systems. Typically, the raw signals on the field wiring are applied to conditioning circuits which provide protection against surges and filtering to remove electromagnetic interference. Also, the conditioning circuits often provide electrical isolation between the field wiring and the processing or control equipment that utilize the conditioned electrical signals. Conditioning also can include translating low level signals, such as those generated by thermocouples, to the high level signals required by the downstream equipment. The conditioning circuits are usually mounted on printed circuit boards with several circuits, depending upon their complexity, to a board. Often the boards will carry potentiometers which are used to manually calibrate and adjust the span of the individual signals. The filters used in the various conditioning circuits have time constants which require a settling interval when making adjustments in the circuits. The time constants for the different circuits can vary widely. While one could simply use the longest time constant to establish a settling time which would be suitable for all the circuits, this practice can greatly extend the time required for calibration in a large system having 200 to 300 circuits. It is preferable therefore, to establish a minimum reasonable settling time for each circuit. A minimum settling time cannot just be set and forgotten, however, because the time constant can change with changes in temperature and over a period of time. In addition, excessive noise on a particular circuit may require an increase in the time constant in a filter circuit. In either case, it has heretofore been necessary for a technician to make a physical change in the circuitry.